WO2009125863A1

WO2009125863A1 - High-strength steel plate excellent in low-temperature toughness, steel pipe, and processes for production of both

Info

Publication number: WO2009125863A1
Application number: PCT/JP2009/057420
Authority: WO
Inventors: 藤城泰志; 坂本真也; 原卓也; 朝日均
Original assignee: 新日本製鐵株式会社
Priority date: 2008-04-07
Filing date: 2009-04-07
Publication date: 2009-10-15
Also published as: EP2264205B1; KR20100105790A; EP2264205A1; JP2009270197A; KR101252920B1; US8110292B2; JP4358900B1; BRPI0911117A2; CN101965414B; CN101965414A; EP2264205A4; US20110023991A1

Abstract

Provided are a high-strength steel plate excellent in low -temperature toughness, a high-strength steel pipe made by using the plate as the base material, and processes for the production of both. The steel plate contains Mo: 0.05 to 1.00% and B: 0.0003 to 0.0100% with Ceq of 0.30 to 0.53 and Pcm of 0.10 to 0.20, and has a metal structure which has an area fraction of polygonal ferrite of 20 to 90% with the balance being a hard phase consisting of either bainite or martensite, or both. The steel plate is produced by conducting, successively, strain-introducing rolling at an initiation temperature of (Ar₃+60°C) or below, an end temperature of Ar₃ or above and a rolling ratio of 1.5 or above, air cooling, and accelerated cooling from a temperature of (Ar₃-100°C) to (Ar₃-10°C) at a rate of 10°C/s or above.

Description

Description Title of Invention

High strength steel plate and pipe excellent in low temperature toughness and manufacturing method thereof Technical Field

The present invention relates to a high-strength steel plate and a steel pipe excellent in low-temperature toughness, particularly suitable for line pipes for transporting crude oil and natural gas. Background art

In recent years, increasing the internal pressure of pipelines has been studied in order to improve the transport efficiency of crude oil and natural gas. Along with this, high strength steel pipes for line pipes are required. In addition, steel pipes for high-strength line pipes are required toughness, deformation performance, and wear resistance. For this reason, steel plates and steel pipes have been proposed, which are mainly composed of bainite and martensite and produce fine ferrules.

For example, see Japanese Patent Laid-Open No. 2 0 0 3-2 9 3 0 78, Japanese Patent Laid-Open No. 2 0 0 3-3 0 6 7 4 9 and Japanese Patent Laid-Open No. 2 0 0 5-1 4 6 4 0 7 . However, these are high-strength steel pipes of the American Petroleum Institute (A P I) standard X I 0 0 (tensile strength of 760 M Pa or more).

On the other hand, API standard X 70 (tensile strength of 5 70 MPa or more) and API standard X 8 0 (tensile strength of 6 25 MPa or more), which have been put to practical use as materials for mainline pipelines. There is also a demand for high-performance steel pipes. In response to this, a method has been proposed in which the heat affected zone (HA Z) of a steel pipe having a base material that generates fine ferritic flaws in the bainite is heat-treated to improve deformation performance and low-temperature toughness. ing. For example, see Japanese Patent Application Laid-Open No. 2 0 0 4-1 3 1 7 9 9. In this way, a method has been proposed for improving the properties such as deformation performance by further generating ferrite based on bainite that has both strength and toughness, steel sheet and steel pipe mainly composed of martensite. Yes. However, recently, the demand for low-temperature toughness is increasing, and base metal toughness at a cryogenic temperature of 160 or below is required. In addition to the base metal, the low temperature toughness of HAZ is also very important. Summary of the Invention

In order to improve HAZ toughness, the carbon equivalent C e Q and cracking susceptibility index P cm are controlled, and B and Mo are added to improve the hardenability. Is effective. However, on the other hand, it becomes difficult to generate ferrules on the base material. In particular, when hardenability is improved by adding B and Mo in combination, the transformation of Ferai will not easily occur. In particular, it was extremely difficult to produce polygonal ferrite by air cooling immediately after the end of hot rolling. In view of such circumstances, the present invention controls the carbon equivalent CeQ and cracking susceptibility index Pcm, and further generates polygonal ferrite on a high-strength steel sheet with improved hardenability by adding B and Mo. Is. In particular, the present invention is intended to improve the low-temperature toughness of the base material, and to provide a high-strength steel pipe using the high-strength steel plate as a base material and a method for producing them.

In the present invention, a ferrite that is not stretched in the rolling direction and has an aspect ratio of 4 or less is referred to as a polygonal ferrite. Here, the aspect ratio is a value obtained by dividing the length of ferrite grains by the width.

Conventionally, B and Mo were added simultaneously, hardenability index C e Q and crack susceptibility index P cm, which is an index of weldability, were controlled within the optimum range, and the metal structure of high-strength steel sheet with improved HAZ toughness The polygonal ferrite It was difficult to generate. In the present invention, the metallographic structure of a steel sheet having a high hardenability component composition is made into a multiphase structure of polygonal ferrite and a hard phase by optimizing the hot rolling conditions. The gist of the present invention is as follows.

(1) By mass%, C: 0.0 1 0 to 0.0 8%, S i: 0.0 1 to 0.5 0%, M n: 0.5 to 2.0%, S: 0 0 0 0 1 to 0. 0 0 5%, T i: 0. 0 0 3 to 0.0. 0 3 0%, Mo: 0. 0 5 to 1.0 0%, B: 0. 0 0 0 3 to 0. 0 1 0%, 0: 0. 0 0 0 1 to 0. 0 0 8% included, P: 0. 0 5 0% or less, A 1: 0. 0 2 0% or less The remainder has a composition composed of iron and inevitable impurities, CeQ determined by the following (Equation 1) is 0.30 to 0.53, and P cm determined by the following (Equation 2) is 0. 10 to 0.20, and the area ratio of polygonal ferrite ridges in the metal structure is 20 to 90%, and the balance is a hard phase consisting of one or both of bainite and martensite. A high-strength steel sheet with excellent low-temperature toughness.

C eq = C + M n / 6 + (N i + C u) / 1 5

+ (C r + M o + V) / 5 (Equation 1)

P cm = C + S i / 3 0 + (M n + C u + C r) / 2 0

+ N i / 6 0 + M o / 1 5 + V / l 0 + 5 B-

)

Here, C, Si, Mn, Ni, Cu, Cr, Mo, V, and B are the content [% by mass] of each element.

(2) The above-mentioned, further comprising one or both of Cu: 0.05 to: I. 5% and Ni: 0.05 to 5.0% by mass%.

(1) A high-strength steel sheet having excellent low temperature toughness.

(3) Further, in mass%, Cr: 0.02 to; .50%, W: 0.01 to 0.50%, V: 0.01 to 0.10%, N b: 0. 0 0 1 to 0.20%, Z r: 0. 0 0 0 1 to 0.0 0 50%, T a: 0. 0 0 0 0 1 to 0. A high-strength steel sheet excellent in low-temperature toughness as described in (1) or (2) above.

(4) Further, in mass%, M g: 0. 0 0 0 1 to 0.0 1 0%, C a: 0. 0 0 0 1 to 0. 0 0 5%, REM: 0. 0 0 0 1 to 0. 0 0 5%, Y: 0. 0 0 0 1 to 0. 0 0 5%, H f: 0. 0 0 0 0 to 0. 0 0 5%, Re: 0. 0 0 0 The high-strength steel sheet according to any one of (1) to (3) above, which contains one or more of 1 to 0.05%.

(5) The high-strength steel sheet according to any one of (1) to (4) above, wherein the area ratio of the polygonal ferrite of the metal structure is 20 to 80%.

(6) A high-strength steel pipe excellent in low-temperature toughness, characterized in that the base material is the steel sheet described in any of (1) to (4) above.

(7) The steel slab comprising the component described in any of (1) to (4) above is reheated to 9500 or more, hot rolled, and started as the final stage of the hot rolling. Strain-introducing rolling is performed at a temperature of A r ₃ + 60 or less, an end temperature of A r ₃ or more, and a reduction ratio of 1.5 or more, and then air-cooled, A r 3-1 0 0: ~ A r ₃ — A high-strength steel sheet with excellent low-temperature toughness characterized by accelerated cooling from a temperature of 10 to a temperature of B s or less determined by the following (Equation 3) at a cooling rate of 1 OZ s or more. Production method.

B s (in) = 8 3 0-2 7 0 C-9 0 M n-3 7 N i-7 0 C r

— 8 3 M o · · · (Formula 3)

Here, C, Mn, Ni, Cr, and Mo are the contents [% by mass] of each element. (8) A low temperature characterized in that the steel sheet produced by the method described in (7) above is formed into a tubular shape in the UO process, the butt portion is welded to the submerged door from the inner and outer surfaces, and then expanded. A manufacturing method for high strength steel pipes with excellent toughness. Brief Description of Drawings

Figure 1 shows the relationship between hot working temperature and polygonal ferrite area ratio.

Figure 2 shows the relationship between the water cooling start temperature and the polygonal ferrite area ratio.

Fig. 3 shows the relationship between the polygonal ferrite area ratio, toughness, and strength. BEST MODE FOR CARRYING OUT THE INVENTION

In order to improve the toughness of high-strength steel sheets, particularly to secure the toughness at a very low temperature of 1-40 and −60, it is necessary to refine crystal grains. However, it is difficult to refine a metal structure composed of bainite and martensite by rolling. In addition, toughness improves when soft ferrite is generated. However, it was found that toughness decreases when hot rolling is performed in a temperature range where austenite and ferrite coexist to produce machined ferrite.

Therefore, the present inventors have directed a method for improving the low-temperature toughness of high-strength steel sheets by generating polygonal ferrite at the time of cooling at high temperature after completion of hot rolling. However, it is difficult to generate polygonal ferrite with a high-strength steel plate with high hardenability to ensure the strength and toughness of HAZ.

In order to generate polygonal ferrite, the steel sheet is hot-rolled directly. It is effective to increase the dislocation density of unrecrystallized austenite after air cooling. The inventors first studied the rolling conditions in the temperature range where the metal structure is austenite and does not recrystallize, that is, in the non-recrystallized region.

% By mass: C: 0.0 1 to 0.0 8%, S i: 0.0 1 to 0.5 0%, M n: 0.5 to 2.0%, S: 0.0 0 0 0 1 to 0. 0 0 5%, T i: 0. 0 0 3 to 0.0 3 0%, ○: 0. 0 0 0 1 to 0.0. 0 0 8% included, P: 0. 0 5 0 %, A 1: 0. 0 20% or less, Mo content is 0.05 ~; L. 0 0%, B content is 0.00 0 3 ~ 0. The carbon equivalent CeQ is 0.30 to 0.53 and the crack susceptibility index Pcm is 0.1 to 0 to 0.20. Steel smelted was produced and forged to produce steel slabs.

Next, a test piece having a height of 12 mm and a diameter of 8 mm was cut from the obtained steel slab and subjected to thermomechanical processing simulating hot rolling. As a thermomechanical treatment, a single process with a rolling reduction ratio of 1.5 was performed, cooled by 0.2 at 0.2 corresponding to air cooling, and further accelerated at 15: Z s corresponding to water cooling. . In order to avoid the formation of machining ferrite, the machining temperature was set to a temperature equal to or higher than the transformation temperature Ar ₃ during cooling. The transformation temperature A r ₃ during cooling was obtained from the thermal expansion curve. After the thermomechanical treatment, the area ratio of the polygonal ferrite ridge of the test piece was measured. In addition, polygonal ferrite was used for ferrite with an aspect ratio of 1 to 4 that was not stretched in the rolling direction.

The temperature at which accelerated cooling at 1 5: Z s corresponding to water cooling starts is A r ₃ — 90, and A r 3-7 O:, A r ₃ — 4 0. We examined the conditions under which polygonal ferrite was generated by changing the processing temperature. The results are shown in Figure 1. Figure 1 plots the area ratio of polygonal ferrule 卜 against the difference between the processing temperature and A r _3. Yes, “O”, “Mouth”, “△” indicate the start temperature of accelerated cooling, respectively, A r ₃ — 90, A r ₃ — 7 0, A r ₃ — 4 0 ^: This is the result. As shown in Fig. 1, it was found that polygonal ferrite with an area ratio of 20% or more was generated if the hot working temperature was set to Ar ₃ + 60 t: or less.

Furthermore, using a hot rolling mill, the relationship between the acceleration start temperature and the area ratio of polygonal ferrite and the relationship between the area ratio of polygonal ferrite and toughness were investigated. In the hot rolling, the reheating temperature was set to 10 50, the number of passes was set to 20 to 3 times, the rolling was finished at Ar ₃ or more, air cooling was performed, and water cooling was performed as accelerated cooling.

The final process of hot rolling, that is, rolling from 8 1 " ₃ + 60 to below is called strain-introducing rolling. The rolling ratio from below to the end at Ar ₃ + 60, ie, strain-introducing rolling. The rolling reduction ratio was set to 1.5 or more, and after air cooling, water cooling (accelerated cooling) was started from various temperatures The number of passes of strain-introducing rolling was 4 to 20 times.

The area ratio of the polygonal ferrite plate of the obtained steel sheet was measured using an optical microscope, and a tensile test and a drop weight test (referred to as Drop Weight Tear Test, DWT) were performed. Tensile properties were evaluated using A PI standard test pieces. DWT T was performed at −60, and the ductile fracture surface ratio (referred to as Shear Area, SA) of the crack was determined.

Figure 2 shows the relationship between the accelerated cooling start temperature and the area ratio of polygonal ferrite. From Fig. 2, if the starting temperature of accelerated cooling after hot rolling is Ar 3-100 0 ~ Ar ₃ — 1 0, the area ratio of the polygonal ferrite of the steel sheet is 20 ~ 9 It was found to be 0%. That is, after the hot rolling is completed, when the air cooling is performed from a temperature of A r ₃ or higher to a temperature within the range of A r ₃ — 1 0 0: to A r ₃ — 1 0, the area ratio 2 0 to 9 0 % Polygonal ferrite can be generated. Figure 3 shows the relationship between the area ratio of polygonal ferrite, the tensile strength, and the ductile fracture surface area SA at 1 6 Ot :. From Fig. 3, it can be seen that extremely good low-temperature toughness can be obtained if the area ratio of polygonal ferrule is 20% or more. From Fig. 3, it is clear that the area ratio of polygonal ferrite 卜 needs to be 90% or less in order to secure a tensile strength of 5 7 OMPa or more corresponding to X70. Furthermore, as shown in FIG. 3, in order to secure a tensile strength of 6 25 MPa or more corresponding to X 80, it is preferable that the area ratio of the polygonal ferrule is 80% or less.

As described above, the present inventors have found that in order to secure polygonal ferrite, it is important to introduce strain due to rolling in an unrecrystallized region when performing hot rolling. The present inventors have conducted further detailed studies and obtained the following findings to complete the present invention.

In hot rolling, it is important to secure a reduction ratio below A r ₃ +60. Therefore, it is necessary to perform strain-introducing rolling as the final hot rolling process. The strain-introduced rolling is Ar ₃ +60 in hot rolling, and is a pass to the end of rolling. At least one pass is required, and multiple passes may be used. In order to generate polygonal ferrite by air cooling after hot rolling, the reduction ratio of strain-introducing rolling should be 1.5 or more. The rolling reduction ratio of the strain-introduced rolling is the ratio of the thickness of Ar ₃ +60: and the thickness after the rolling.

After rolling, air-cooled to generate polygonal ferrite, and then accelerated and cooled at a cooling rate of 10 s or more to improve the strength by bainitic transformation. In addition, in order to ensure the strength, it is necessary to stop the accelerated cooling below the vane generation temperature B s.

Hereinafter, the steel sheet of the present invention will be described in detail. “%” Means “% by mass”. C: 0.0 1 to 0.0 8%

C is an element that improves the strength of steel, and in order to form a hard phase composed of one or both of bainite and martensite in the metal structure, addition of 0.1% or more is necessary. In the present invention, in order to achieve both high strength and high toughness, the C content is set to 0.08% or less.

S i: 0.0 1 to 0.5 0%

S i is a deoxidizing element, and in order to obtain an effect, addition of 0.0 1% or more is necessary. On the other hand, if containing Si exceeding 0.50%, the toughness of HA Z deteriorates, so the upper limit is made 0.5%.

M n: 0.5 to 2.0%

M n is an element that enhances hardenability, and it is necessary to add 0.5% or more in order to ensure strength and toughness. On the other hand, if the Mn content exceeds 2.0%, the toughness of HA Z is impaired. Therefore, the content of M n is 0.5 to 2.0%.

P: 0.05 0% or less

P is an impurity, and if it contains more than 0.050%, the toughness of the base material is significantly lowered. In order to improve the toughness of HA Z, the P content is preferably 0.02% or less.

S: 0. 0 0 0 1 to 0.0. 0 5%

S is an impurity, and if it exceeds 0.05%, coarse sulfides are produced and the toughness is lowered. In addition, when Ti oxide is finely dispersed in the steel sheet, MnS precipitates and causes intragranular transformation, improving the toughness of the steel sheet and HAZ. In order to obtain this effect, it is necessary to contain S in an amount of 0.001% or more. Further, in order to improve the toughness of HA Z, it is preferable to set the upper limit of the S amount to 0.03%.

A 1: 0.0 2 0% or less

A 1 is a deoxidizer, but it suppresses the formation of inclusions and suppresses the formation of steel plates and HAZ. In order to increase the toughness of the steel, the upper limit must be 0.020%. By limiting the content of A 1, it is possible to finely disperse the Ti oxide that contributes to the intragranular transformation. In order to promote the formation of intragranular transformation, it is preferable that the content of 8 1 is not more than 0.010%. A more preferred upper limit is 0.0 0 8%.

T i: 0.0.03 to 0.0.30%

T i is an element that forms a nitride of T i that contributes to the refinement of the grain size of the steel sheet and HA Z, and it is necessary to add 0.003% or more. On the other hand, if Ti is contained excessively, coarse inclusions are formed and the toughness is impaired, so the upper limit is made 0.030%. In addition, when Ti oxides are finely dispersed, they effectively act as nuclei for intragranular transformation.

If the amount of oxygen at the time of adding T 1 is large, a coarse oxide of T 1 is formed. Therefore, during steelmaking, it is preferable to deoxidize with Si and M n to reduce the amount of oxygen. . In this case, since the oxide of A 1 is easier to form than the oxide of T i, it is not preferable to contain an excessive amount of A 1.

B: 0. 0 0 0 3 to 0.0. 0 1 0%

B is an important element that significantly enhances hardenability and suppresses the formation of coarse grain boundary ferrite in HA Z. In order to obtain this effect, it is necessary to add B 0.003% or more. On the other hand, when B is added excessively, coarse B N is produced, and in particular, the toughness of HA Z is lowered. Therefore, the upper limit of B content is set to 0.0 10%.

M o: 0.0 5 ~: 1. 0 0%

Mo is an element that remarkably enhances the hardenability especially by the combined addition with B, and 0.05% or more is added to improve strength and toughness. On the other hand, Mo is an expensive element, and it is necessary to set the upper limit of the addition amount to 1.0%.

0: 0. 0 0 0 1 to 0. 0 0 8% O is an impurity, and the upper limit of the content must be 0.08% in order to avoid a decrease in toughness due to the formation of inclusions. In order to produce oxides of ίί that contribute to intragranular transformation, the amount of ◯ remaining in the steel at the time of forging is set to not less than 0.0 0 0 1%.

Further, one or more of Cu, Ni, Cr, W, V, Nb, Zr, and Ta may be added as elements for improving strength and toughness. . In addition, when the content of these elements is less than the preferred lower limit, they do not have a particularly bad influence, and can be regarded as impurities.

Cu and Ni are effective elements that increase the strength without impairing toughness. To obtain the effect, the lower limit of the Cu content and the Ni content must be 0.05% or more. Is preferred. On the other hand, the upper limit of the Cu content is preferably 1.5% in order to suppress the occurrence of cracks during heating and welding of the steel slab. If Ni is contained excessively, weldability is impaired, so the upper limit is preferably made 5.0%.

Note that Cu and Ni are preferably included in combination in order to suppress the occurrence of surface flaws. Further, from the viewpoint of cost, it is preferable that the upper limit of Cu and Ni is 1.0%.

Cr, W, V, Nb, Zr, and Ta are elements that generate carbides and nitrides and improve the strength of steel by precipitation strengthening, and contain one or more. You may let them. In order to increase the strength effectively, the lower limit of the Cr amount is 0.02%, the lower limit of the \ ¥ amount is 0.01%, the lower limit of the V amount is 0.01%, and the Nb amount The lower limit is preferably 0.0 0 1%, and the lower limits of the Zr amount and the Ta amount are both preferably 0.0 0 0 1%. On the other hand, if one or both of Cr and W are added excessively, the hardenability will improve and the strength will be increased and the toughness may be impaired. Therefore, the upper limit of Cr content is 1.50% and the upper limit of W content. Is preferably 0.5%. . In addition, excessive addition of one or more of V, Nb, Zr, and Ta may cause carbides and nitrides to become coarse and impair toughness, so the upper limit of V content is set to 0. It is preferable that the upper limit of the amount of 10%, the amount of Nb is 0.20%, and the upper limit of the amount of Zr and Ta is both 0.05%.

Furthermore, in order to control the form of inclusions and improve toughness, one or more of Mg, Ca, REM, Y, Hf, and Re may be added. . In addition, these elements can be regarded as impurities because they do not have an adverse effect when the content is less than the preferred lower limit.

Mg is an element that has an effect on the refinement of oxides and the suppression of sulfide morphology. In particular, fine Mg oxides act as nuclei for intragranular transformation and also suppress the coarsening of the particle size as pinning particles. In order to obtain these effects, it is preferable to add 0.001% or more of Mg. On the other hand, if Mg in an amount exceeding 0.010% is added, a coarse oxide is formed, which may reduce the toughness of HA Z. % Is preferable.

C a and REM are useful for controlling the morphology of sulfides, suppress the formation of sulfides and the formation of M n S that stretches in the rolling direction, and the properties in the thickness direction of steel materials, especially lamellar resistance. It is an element that improves the properties. In order to obtain this effect, it is preferable that the lower limits of the Ca content and the REM content are both 0.0 0 0 1%. On the other hand, one or both of Ca and REM, when the content exceeds 0.05%, the oxide increases, the fine Ti-containing oxide decreases, and the formation of intragranular transformation is inhibited. Therefore, it is preferable to set the content to 0.05% or less.

Y, H f, and R e are elements that exhibit the same effects as Ca and REM, and if added in excess, they may inhibit the formation of intragranular transformation. Therefore, the preferred range of the amount of Y, H f, and Re is 0 0 0 0 1 to 0. 0 0 5%.

Furthermore, in the present invention, in order to secure the hardenability of HAZ and enhance the toughness, the contents of C, M n, Ni, Cu, Cr, Mo, and V [mass%] The carbon equivalent C eQ of the following (formula 1) calculated from the equation (3) is set to 0.30 to 0.53. The carbon equivalent C eQ is known to correlate with the maximum hardness of the weld and is a value that can be used as an index of hardenability and weldability.

C ea = C + M n / 6 + (N i + C u) / 1 5

+ (C r + Mo + V) / 5---(Formula 1)

In order to ensure the low temperature toughness of the steel sheet and HAZ, it is calculated from the contents [mass%] of C, Si, Mn, CuCr, Ni, Mo, V, and B. In the following (Equation 2), the cracking sensitivity index P cm is set to 0.10 to 0.20. The crack susceptibility index P cm is known as a coefficient that can be used to estimate the susceptibility to cold cracking during welding, and is a value that serves as an index of hardenability and weldability.

P cm = C + S 1/3 0 + (M n + C u + C r) / 2 0

+ N i / 60 + Mo / 1 5 + V / l 0 + 5 B-(Formula 2) In addition, the selectively contained elements, Ni, Cu, Cr, V are If it is less than the above-mentioned preferred lower limit, it is an impurity, so in the above (formula 1) and (formula 2), it is calculated as 0.

The metal structure of the steel sheet is a composite structure containing polygonal ferrite and hard phase. Polygonal ferrite is generated at a relatively high temperature during air cooling after hot rolling. Polygonal ferrite has an aspect ratio of 1 to 4, and is a processed ferrite that is rolled and stretched, and a fine ferrite that is formed at a relatively low temperature during accelerated cooling and has insufficient grain growth. Are distinguished.

The hard phase is either one or both of paynite and martensite Organization. In the optical microstructure of the steel sheet, residual austenite and MA may be included as the remainder of the polygonal ferrite and the vine and martensite.

The area ratio of Polygonal Ferai is 20% or more. As described above, a steel sheet having a component composition with improved hardenability has a good balance between strength and toughness by generating polygonal ferrite and using the balance as the hard phase of bainite and martensite. In particular, by setting the area ratio of the polygonal ferrule to 20% or more, as shown in Fig. 3, the low temperature toughness is remarkably improved. As a result of DWTT at 160, SA is 85% This can be done.

On the other hand, in order to ensure the strength, the area ratio of polygonal X-ray rice must be 90% or less. As shown in Fig. 3, if the area ratio of L ¾ gonal ferrule 卜 is 90% or less, X 7

A tensile strength corresponding to 0 or more can be secured. Furthermore, in order to increase the strength and secure a tensile strength corresponding to X 80 or more, it is preferable that the area ratio of the polygonal ferri iron is 80% or less.

Further, the remainder of the polygonal ferrite is a hard phase composed of one or both of bainai and martensite. The area ratio of the hard phase is 2 0 90% because the area ratio of polygonal ferrule is 1 0

8 0%. On the other hand, for example, if a milling ferrit with a finish temperature lower than A r 3 and an aspect ratio exceeding 4 is generated, the toughness decreases.

In the present invention, a polygonal ferrite is a white rounded lump that has an aspect ratio of 14 and does not contain precipitates such as coarse cementite MA in the grain in an optical microscope. Observed as an organization. Here, the aspect ratio is the value obtained by dividing the length of the ferrite grains by the width. Bainite is defined as a structure in which carbides are precipitated between the laths or block ferrite, or a structure in which carbides are precipitated in the laths. Furthermore, martensite is a structure in which carbides are not precipitated between the laths or within the laths. Residual austenite is austenite that remains without transformation of austenite soot generated at high temperature.

Next, the manufacturing method for obtaining the steel plate of this invention is demonstrated. The above-mentioned components have improved hardenability in order to improve the toughness of HAZ, and in order to improve the low temperature toughness of the steel sheet, it is necessary to control hot rolling conditions and generate ferrite. is necessary. In particular, according to the present invention, even when it is difficult to increase the reduction ratio in the hot rolling process, such as a steel sheet having a thickness of 20 mm or more, a reduction ratio at a relatively low temperature is ensured. As a result, a ferrite can be generated.

First, steel is melted in the steelmaking process, and then forged into billets. Steel melting and forging can be carried out by conventional methods, but continuous forging is preferred from the viewpoint of productivity. The billet is reheated for hot rolling.

The reheating temperature during hot rolling is 9 50 or more. This is because hot rolling is performed at a temperature at which the steel structure becomes an austenite single phase, that is, in the austenite region, and the crystal grain size of the base steel sheet is made fine. Although the upper limit is not specified, in order to suppress the coarsening of the effective crystal grain size, it is preferable to set the reheating temperature to 1 2 500 or less. In order to increase the area ratio of polygonal ferrite, it is preferable to set the upper limit of the reheating temperature to 10 50 or less.

The reheated slab is subjected to multiple passes of hot rolling while controlling the temperature and reduction ratio, and after completion, it is air-cooled and accelerated. In addition, hot rolling must be completed at an Ar ₃ temperature or higher at which the base metal structure becomes an austenite single phase. This is hot rolling at less than A r ₃ temperature This is because machining ferrite is generated and toughness decreases. In the present invention, it is extremely important to perform strain-introducing rolling as the final step of hot rolling. This is because, after the rolling is completed, a large amount of distortion, which is a site for generating polygonal ferrite, is introduced into the non-recrystallized austenite. Strain-introduced rolling is defined as the path from A r ₃ +60: below to the end of rolling. The starting temperature of strain-introducing rolling is the temperature of the first pass below A r ₃ +60. The starting temperature of the strain-introducing rolling is preferably a lower temperature of Ar 3 +40 or lower.

The reduction ratio of strain-introduced rolling is 1.5 or more in order to generate polygonal ferrite during air cooling after hot rolling. In the present invention, the reduction ratio of strain-introducing rolling is the thickness at Ar ₃ +60: or the thickness at the starting temperature of strain-introducing rolling is divided by the thickness after the end of hot rolling. It is a ratio. Although the upper limit of the reduction ratio is not specified, it is usually 12.0 or less considering the thickness of the steel slab before rolling and the thickness of the base steel plate after rolling. In order to increase the area ratio of the polygonal ferritic iron of the steel sheet having a component composition with improved hardenability, it is preferable to set the rolling reduction ratio of the strain-introducing rolling to 2.0 or more.

Note that recrystallization rolling and non-recrystallization rolling may be performed before the strain-introducing rolling. The recrystallization rolling is rolling in a recrystallization region exceeding 90 °, and the non-recrystallization rolling is rolling in the following non-recrystallization region at 90 °. Recrystallization rolling may be started immediately after the slab is extracted from the heating furnace, so the starting temperature is not specified. In order to refine the effective crystal grain size of the steel sheet, the reduction ratio of recrystallization rolling is preferably 2.0 or more. In addition, after rolling, air cooling and accelerated cooling are performed. For the area ratio to produce 2 0-9 0% of polygonal Blow wells, it is necessary to air-cooled to eight 1 _"3 of less than the temperature Thus, the accelerated cooling, A r 3 -. 1 0 0 to ~ A r ₃ — 1 Need to start at a temperature in the range of 0 There is. In order to suppress the formation of pearlite and cementite and to secure tensile strength and toughness, the cooling rate for accelerated cooling must be at least l O ^ Z s.

Accelerated cooling suppresses the formation of pearlite and cementite, and in order to generate a hard phase consisting of one or both of bainey 卜 and martensite 卜, the stop temperature is set below B s in (Equation 3). There is a need. B s is the bainitic transformation start temperature, and it is known that it can be obtained from the contents of C, M n, Ni, Cr and Mo by (Equation 3). Payne 卜 can be generated by accelerated cooling to temperatures below B s.

B s (in) = 8 3 0-2 7 0 C-90 M n-3 7 N i-70 C r — 8 3 M o · · · (Equation 3)

The lower limit of the water cooling stop temperature is not stipulated, and it may be cooled to room temperature. However, considering productivity and hydrogen defects, it is preferable to set the temperature to 1550 or higher. Example

Steel having the composition shown in Table 1 was melted into steel pieces having a thickness of 240 mm. These steel slabs were hot-rolled under the conditions shown in Table 2 and cooled to produce steel plates. The Ar ₃ of each steel type was obtained by measuring the thermal expansion after cutting a test piece having a height of 12 mm and a diameter of 8 mm from the melted steel piece and subjecting it to a heat treatment simulating hot rolling. table 1

* CeQ = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5

* Pci = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B * Ingredient empty means no additive

* Underline in the table means outside the scope of the present invention

Table 2

* The reduction ratio is (sheet thickness before the start of strain-introducing rolling) / (final sheet thickness). * Rolling end temperature, water cooling start temperature, water cooling stop temperature are differences from Ar ₃ <* Underline in the table means outside the scope of the present invention

The microstructure of the central part of the steel sheet was observed with an optical microscope, and the area ratios of the polygonal ferrite and the remainder of the vein and martensite were measured. In addition, press notch test specimens were prepared from steel plates in accordance with API, 5 L 3, ASTM, E 4 36, with the plate width direction as the longitudinal direction and the notches parallel to the plate thickness direction. Produced. DWT T was performed at 1 60 and SA was obtained. Tensile properties were evaluated using API standard test pieces. The results are shown in Table 3.

Table 3

* Underline in the table means outside the scope of the present invention Manufacturing No. 1 to 3, 6, 7, 10, 12, 14, 16 to 19 are examples of the present invention, and a polygonal ferrule with an aspect ratio of 1 to 4 is an area. The rate is 20 to 90%. These are steel sheets excellent in low-temperature toughness that satisfy the strength of X 70 or more, further X 80 or more, and the SA in DWT T is 85% or more.

These steel plates were piped in the U0 process, the butted parts were submerged arc welded from the inner and outer surfaces, and expanded to produce steel pipes. The structure of these steel pipes was the same as that of the steel plate, the strength was 20 to 30 MPa higher than that of the steel plate, and the low temperature toughness was equivalent to that of the steel plate.

On the other hand, production No. 4 is an example in which the start temperature of accelerated cooling is low, the area ratio of ferrite increases, and the strength decreases. Production No. 5 is an example in which the cooling rate of accelerated cooling is slow, a hard phase for securing the strength cannot be obtained, and the strength is lowered. Production No. 8 has a rolling finish temperature lower than A r ₃ , so a machining ferrite with a aspect ratio exceeding 4 is generated, polygonal ferrite is reduced, and low-temperature toughness is reduced. This is an example.

In production No. 8, the polygonal ferrite and the remainder of the hard phase are ferrite with an aspect ratio of more than 4.

Production No. 9, 13 and 15 have higher accelerated cooling start temperatures, and Production No. 11 has a lower strain reduction rolling reduction, resulting in insufficient ferrite formation and toughness. This is an example of a decline.

In addition, the production No 2 0 to 2 2 is a comparative example whose chemical component is outside the scope of the present invention. Production No. 20 has a small amount of B, and Production No. 22 does not contain Mo. Therefore, the production conditions of the present invention are examples in which the polygonal ferrite is increased and the strength is lowered. Production No 2 1 is an example in which the amount of Mo is large, and the area ratio of polygonal ferrite is low even under the production conditions of the present invention, and the toughness is lowered. Industrial applicability

According to the present invention, in the metal structure of a high-strength steel sheet having a component composition in which the carbon equivalent C ec i and the cracking susceptibility index P cm are controlled and B and Mo are further added and the hardenability is enhanced,卜 can be generated. As a result, the strength and HAZ toughness are improved, and the low-temperature toughness is extremely excellent, and the high-strength steel pipe whose metal structure consists of polygonal ferrite and hard phase, and the high-strength steel pipe using this as a base material, And it becomes possible to provide their manufacturing methods, and the industrial contribution is very remarkable.

Claims

Scope of claim Claim 1.

C: 0 • 0 1 to 0.08%,

S1: 0 0 1 0.5.50%,

M n: 0 • 5 2.0%,

S: 0 • 0 0 0 1 to 0.0 0 5 / o,

T i: 0 0 0 3 to 0. 0 3 0 0 /

/ o,

M o: 0 • 0 5 1. 0 0%

B: 0 0 0 0 3 to 0.0 0 1 0 / ο,

○: 0 0 0 0 1 to 0. 0 0 8 0 /

/ ο

Including

P: 0 • 0 5 0% or less,

A 1: 0 0 2 0% or less

And the balance is a component composition consisting of iron and inevitable impurities, CeQ determined by the following (formula 1) is 0.30 to 0.53, and Pcm determined by the following (formula 2) 0.10 to 0.20, the area ratio of polygonal ferrite ridges in the metal structure is 20 to 90%, and the balance is a hard material composed of one or both of bainite and martensite. A high-strength steel sheet with excellent low-temperature toughness characterized by being a phase.

C eq = C + M n / 6 + (N i + C u) ノ 15 + (C r + o + V) c (Equation 1)

P cm = C + S 1/3 0 + (M n + C u + C r) / 2 0

+ N i / 6 0 + M o / 1 5 + V / l 0 + 5 B-(Equation 2) where C, S i, M n, N i, Cu, Cr, Mo, V, and B are the content [% by mass] of each element.

Claim 2. Further, in mass%, C u: 0.0 5 ~: 1.5%,

N i: 0. 0 5 to 5. 0%

The high-strength steel sheet excellent in low-temperature toughness according to claim 1, comprising one or both of the following.

Claim 3. Further, in mass%,

C r: 0.0 2 to: I. 50%

W: 0.0 1 to 0.5 0%,

V: 0.0 1 to 0.1 0%,

N b: 0.0 0 1 to 0.20%,

Z r: 0. 0 0 0 卜 0. 0 5 0%,

T a: 0. 0 0 0 1 to 0. 0 5 0%

The high-strength steel sheet excellent in low temperature toughness according to claim 1 or 2, characterized by containing one or more of them.

Claim 4. Furthermore, in mass%,

M g: 0. 0 0 0 1 to 0.0. 0 1 0%,

C a: 0.0 0 0 1 to 0.0 0 5%,

R E M: 0. 0 0 0 1 ~ 0.0 0 0 5%,

Y: 0. 0 0 0 1 to 0.0 0 5%,

H f: 0. 0 0 0 1 to 0.0. 0 5%,

R e: 0. 0 0 0 1 to 0.0. 0 0 5%

The high-strength steel sheet according to any one of claims 1 to 3, wherein one or more of them are contained.

5. The high-strength steel sheet according to any one of claims 1 to 4, wherein the area ratio of the polygonal ferrite ridges having a metal structure is 20 to 80%.

6. A high strength steel pipe excellent in low temperature toughness, characterized in that the base material is the steel sheet according to any one of claims 1 to 5. Claim 7. The steel slab comprising the component according to any one of claims 1 to 4 is reheated to 9500 or more, hot rolled, and started as a final step of the hot rolling. Strain-introducing rolling with temperature of Ar ₃ + 60 or less, end temperature of Ar ₃ or more, and reduction ratio of 1.5 or more, followed by air cooling and A r ₃ — 1 0 0 to 8 1 "High-strength steel sheet with excellent low-temperature toughness characterized by accelerated cooling from a temperature of ₃ _ 10 to a temperature of B s or less determined by (Equation 3) below at a cooling rate of l O ^ Z s or higher Manufacturing method.

B s (in) = 8 3 0-2 7 0 C-9 0 M n-3 7 N i-7 0 C r

— 8 3 M o · · · (Formula 3)

Here, C, Mn, Ni, Cr, and Mo are the contents [% by mass] of each element.

Claim 8. The steel sheet produced by the method according to claim 7 is formed into a tubular shape in the UO process, the butt portion is submerged arc welded from the inner and outer surfaces, and then expanded to a low temperature toughness. An excellent method for manufacturing high-strength steel pipes.